The ability to switch the electrical conductivity of organic polymers such as polythiophene, polypyrrole, and polyaniline between states of higher (ON state) and lower (OFF state) conductance has spurred research into the use of these materials as active components in a variety of devices, including transistors, memories, and sensors. Polyaniline is unique amongst the common conducting polymers because switching between the ON and OFF states can be achieved by protonation-deprotonation as well as by the more common oxidation-reduction mechanism. Thus, conversion of the neutral, emeraldine base (EB) of PANT into its positively charged, emeraldine salt (ES) form is accompanied by a nine to ten order of magnitude increase in conductivity (A. G. MacDiamid (2001) Angew. Chem. Int. Ed. 40, 2581-2590: ““Synthetic metals”: A novel role for organic polymers (Nobel Lecture)”).
Conducting polymers have been used in microelectrochemical devices, which are electronic devices using adjacent microelectrodes connected with polymer that mimic the fundamental characteristics of the analogous solid-state devices. The first microelectrochemical device having properties analogous to a solid-state field effect transistor was described by Wrighton et al. in 1984 (H. S. White, G. P. Kittlesen, M. S. Wrighton (1984) J. Am. Chem. Soc. 106, 5375-5377: “Chemical derivatization of an array of three gold microelectrodes with polypyrrole: Fabrication of a molecule-based transistor”; G. P. Kittlesen, H. S. White, M. S. Wrighton (1984) J. Am. Chem. Soc. 106, 7389-7396: “Chemical derivatization of microelectrode arrays by oxidation of pyrrole and N-methylpyrrole: Fabrication of molecule-based electronic devices”). The following year, Wrighton et al. demonstrated polyaniline-based devices with diode-like and transistor-like properties wherein a large change (>106) in electronic conductivity of polyaniline with change in electrochemical potential was the basis of the electronic device function (E. W. Paul, A. J. Ricco, M. S. Wrighton (1985) J. Phys. Chem. 89, 1441-1447: “Resistance of polyaniline films as a function of electrochemical potential and the fabrication of polyaniline-based microelectronic devices”). Despite their simplicity and low cost, microelectrochemical devices have not been very successful commercially except for analytical applications such as chemical or biological sensing. The main disadvantage of microelectrochemical devices compared to their solid-state counterparts is their slower speed of operation. Microelectrochemical transistors are intrinsically slower than solid-state transistors because switching requires the diffusion of ions rather than electrons. Thus, polyaniline-based transistors with ˜1.2 μm channel length and polymer film thickness of the order of a few micrometers could amplify electrical signals up to 102-103 Hz (E. P. Lofton, J. W. Thackeray, M. S. Wrighton (1986) J. Phys. Chem. 90, 6080-6083: “Amplification of electrical signals with molecule-based transistors: Power amplification up to a kilohertz frequency and factors limiting higher frequency operation”], while a reduction in channel length to 0.05-0.1 μm provided transistors that were operational up to ˜104 Hz (E. T. T. Jones, O. M. Chyan, M. S. Wrighton (1987) J. Am. Chem. Soc. 109, 5526-5528: “Preparation and characterization of molecule-based transistors with a 50-nm source-drain separation with use of shadow deposition techniques: Toward faster, more sensitive molecule-based devices”). In contrast, solid-state bipolar transistors are readily capable of switching at frequencies>109 Hz (S. M. Sze, K. K. Ng (2007) Physics of Semiconductor Devices, Wiley-Interscience).
In recent years, electrically rewritable memory cells that operate on the basis of the displacement of mobile dopant ions in polymeric semiconductors under an applied electric field were developed. Terms such as “dopant-configurable”, “electrochemical doping”, and “dynamic doping” have been used to describe such devices. Smits et al. first described a system comprising polythiophene as the polymeric semiconductor and lithium triflate as the source of mobile ions (J. H. A. Smits, S. C. J. Meskers, R. A. J. Janssen, A. W. Marsman, D. M. de Leeuw (2005) Adv. Mater. 17, 1169-1173: “Electrically rewritable memory cells from poly(3-hexylthiophene) Schottky diodes”). Later, the same group described a system comprising a block copolymer of thiophene oligomer and poly(ethylene oxide) as the polymeric semiconductor and sodium chloride as the source of mobile ions (F. Verbakal, S. C. J. Meskers, R. A. J. Janssen (2006) Chem. Mater. 18, 2707-2712: “Electronic memory effects in a sexithiophene-poly(ethylene oxide) block copolymer doped with NaCl. Combined diode and resistive switching behavior”). In that same year, Patil et al. reported two systems comprising a substituted poly-p-phenylene vinylene compound as the polymeric semiconductor and either RbAg4I5 (a solid inorganic electrolyte) or the platinum salts of triflate or hexafluorophosphate as the source of mobile ions (S. Patil, Q. Lai, F. Marchioni, M. Jung, Z. Zhu, Y. Chen, F. Wudl (2006) J. Mater. Chem. 16, 4160-4164: “Dopant-configurable polymeric materials for electrically switchable devices”). Although such devices are potential candidates for non-volatile memory and configurable logic applications, none of them are used as such commercially. One limitation is slow operation speed due to the fact that the mobile dopant ions are relatively large and the devices rely on migration of anions as well as cations. This limitation can be avoided or minimized by using protons as the sole mobile dopant ion.
Due to the sensitivity of its conductivity to protons, polyaniline has been used as a component of chemical and biochemical sensors that detect analytes by changes in film resistance (“chemiresistors”). However, films of polyaniline prepared by the usual solution methods or by electropolymerization generally suffer from slow response times and/or difficulty in achieving reproducible results. Methods have been developed to try to circumvent these shortcomings, such as the use of polyaniline “nanofibers” (D. Li, J. Huang, R. B. Kaner (2009) Acc. Chem. Res. 42, 135-145: “Polyaniline nanofibers: A unique polymer nanostructure for versatile applications”), but these methods involve additional steps. Furthermore, the films thus obtained are usually already acid-doped, so that de-doping is required if the sensor is intended to detect acids or acid-forming compounds. The antennae of RFID tags have also been coated with polyaniline to provide a new type of sensing device, but the polyaniline was acid-doped (R. A. Potyrailo, C. Surman, S. Go, Y. Lee, T. Sivavec, W. G. Morris (2007) J. Appl. Phys. 106, 124902: “Development of radio-frequency identification sensors based on organic electronic sensing materials for selective detection of toxic vapors”).
Accordingly, there is a need in the art to improve the polyaniline-based devices.